This invention relates to power MOSgated devices and more specifically relates to a novel low voltage P-channel MOSFET having a reduced switching loss.
Power Mosgated devices are well known and include such devices as power MOSFETs; IGBTs; gate controlled thyristors and the like. In low voltage applications of such devices, particularly in connection with battery operated portable electronic devices, such as personal computers, cellular telephones and the like, frequently termed wireless systems, careful power management is essential to extend the battery life and its usage between charges.
Power management applications in wireless systems fall generally into two categories. One category is charging the battery from an external DC source. it is important to control both the charging current and voltage correctly for the particular battery technology. This control is accomplished by modulating the duty cycle of a transistor placed between the power source and the battery in well known ways. The second category activates a portion of the system on demand. In this case the transistor is placed between the battery and the load to be activated, such as an RF power amplifier. In some systems, multiple power supply voltages require DC/DC conversion as well. This may be accomplished with well known low dropout linear regulators or buck and boost switching regulators.
Both N-channel and P-channel power MOS transistors as the transistor in the above applications are available. P-channel devices are generally easier to use in these circuits. Thus, when the P-channel MOSFET is placed in the power bus, it can be controlled with a logic input that switches between the power rail and ground. This allows a single uninterrupted ground for the whole system. N-channel devices in the power bus require a gate signal that is boosted to a voltage higher than the bus, which requires extra circuitry.
In the past, the simplicity of a P-channel device came at the price of increased losses. This is because P-channel devices rely on hole conduction, and holes have a lower carrier mobility in silicon than electrons. The on-resistance of the active transistor is proportional to the carrier mobility, and its losses are proportional to the on-resistance, RDSON.
To overcome this limitation, the length of the resistive path should be minimized and the width maximized within the transistor. The number of holes in the path must also be maximized. One way to do this is to lower the maximum voltage rating as much as possible which permits the use of lower resistivity and higher dopant concentration silicon.
Since most batteries operate at only a few volts, a 12V rating is generally more than enough for a transistor in a wireless application. Previously available devices are rated at 20V, and have a reasonably low value of RDSON at 2.5V gate to source. These parts come in various die sizes and package styles, ranging from the Micro 3 (SOT23) up to the SO8. The values listed in the following Table are for single transistors in a package, though the Micro 8 and SO8 packages also have dual versions. The power loss using these devices can be as high as 9% which translates directly into reduced usage.
Vdrop or PdissLoadPackage(as % of 5 VCurrentPart NumberStyleRDSON @ 2.5 Vsupply)500 mAIRLML6302Micro 3 ™ 0.9 Ω9% 1 AIRLMS6702Micro 6 ™ 0.4 Ω8% 2 AIRF7604Micro 8 ™ 0.13 Ω5% 4 AIRF7416SO-80.035 Ω3%
It is known that a low voltage power MOSFET can be made with trench type technology to obtain reduced RDSON, gate to drain capacitance, and to reduce Qg (gate charge). Switching losses are proportional to the product of the device RDSON and Qg so it would be desirable to also reduce RDSON in such devices. The present P-channel trench type power MOSFET uses a P-type substrate with a P-type epitaxial layer thereon. The device channel regions are formed by deep N-type diffusions from the top surface of the epitaxial layer, followed by P-type source diffusions. The voltage is then mostly blocked in the P-type epitaxial layer, resulting in a fairly large resistive drop, and in increased losses in a wireless system. These losses in turn reduce battery life between charges.